Non-Enzymatic Functions of Retroviral Integrase: The Next Target for Novel Anti-HIV Drug Development

Abstract

Integrase (IN) is a retroviral enzyme that catalyzes the insertion of viral DNA (vDNA) into host chromosomal DNA, which is necessary for efficient viral replication. The crystal structure of prototype foamy virus IN bound to cognate vDNA ends, a complex referred to as the intasome, has recently been resolved. Structure analysis of the intasome revealed a tetramer structure of IN that was required for its catalytic function, and also showed the inhibitory mechanism of the IN inhibitor. Genetic analysis of IN has revealed additional non-enzymatic roles during viral replication cycles at several steps other than integration. However, the higher order structure of IN that is required for its non-enzymatic functions remains to be delineated. This is the next major challenge in the field of IN structural biology hoping to be a platform for the development of novel IN inhibitors to treat human immunodeficiency virus type 1 infectious disease.

Keywords: Gemin2; HIV-1; intasome; integrase; pol; reverse transcriptase; reverse transcription.

Figure 1

Schematic diagram of HIV-1 IN. The genetic organization of HIV-1 is shown at the top. HIV-1 IN is encoded by the pol region and composed of 288 amino acids with three structurally distinct domains: an N-terminal domain (NTD); a central catalytic core domain (CCD); and a C-terminal domain (CTD). The CCD contains the highly conserved DDE motif, which is directly involved in the catalytic activities of IN. Overall topology of the CCD is similar to that of ribonuclease H (RNaseH). The NTD, contains a highly conserved HHCC motif, which binds to zinc and folds a helix-turn-helix (HTH) structure. The CTD, consisting of a structure that closely resembles Src homology 3 domains (SH3), possesses sequence- and metal ion-independent DNA binding activity.

Figure 2

Non-enzymatic and enzymatic roles of HIV-1 IN that help establish proviral DNA. After entry into cells, retroviral genomic RNA (vRNA) is reverse transcribed into DNA (vDNA) by RT. Then, vDNA is transported into the nucleus (nuclear import) and finally integrated into host chromosomal DNA (black lines). IN forms a functional tetramer with vDNA ends (intasome) to assist with integration. Subsequent repair processes by host DNA repair machinery establishes the proviral DNA, in which retrovirus DNA is stably integrated into host chromosomal DNA. IN tetramer structures for its enzymatic (integration) and non-enzymatic functions (reverse transcription and nuclear import) and possible involvement of host factors are schematically depicted. The vRNA, vDNA, and host DNA are shown as blue, red, and black lines, respectively. An arrowhead shows the direction of each genome from the 5′- to the 3′-end.